The Diencephalon

The Diencephalon

Diencephalon: -middle structure which is largely embedded in cerebrum. -hidden from surface view. -cavity form greater part of 3rd ventricle. -hypothalamic sulcus extending from interventricular foramen to cerebral aqueduct divides each half of diencephalon into dorsal & ventral parts.

Dorsal part of Diencephalon -Thalamus(dorsal thalamus) -Metathalamus incuding medial & lateral geniculate bodies. -Epithalamus including Pineal body & habenula Ventral part of Diencephalon -Hypothalamus -Subthalamus(ventral thalamus)

Thalamus: -large mass of grey matter situated in lateral wall of 3rd ventricle & in floor of central part of lateral ventricle -has anterior & posterior ends, superior, inferior, medial & lateral surfaces. -anterior end with anterior nuc. is narrow & forms posterior boundary of interventricular foramen. -posterior end is expanded and is known as pulvinar.- overhangs lateral & medial geniculate bodies, superior colliculus with its brachium

-superior surface is divided into a lateral ventricular part which forms floor of central part of lateral ventricles- and medial extra-ventricular part which is covered by telachoridea of 3rd ventricle. it is limited laterally by caudate nucleus, stria terminalis & thalamostriate vein & medially by habenular stria ( stria medullaris thalami) -inferior surface rests on subthalamus & hypothalamus -medial surface forms posterosuperior part of lateral ventricle.- medial surfaces of two thalami are interconnected by an interthalamic adhesion. -lateral surface forms medial boundary of posterior limb of internal capsule.

Structure & Nuclei of Thalamus: White matter: -external medullary lamina covers lateral surface -internal medullary lamina divides thalamus into 3 parts: anterior, medial & lateral. Grey matter: -divided to form several nuclei -anterior nucleus in anterior part. -medial nucleus in medial part.anterior & medial part together represents paleothalamus.

-lateral part of thalamus is larges & represents neothalamus- is divided into lateral nucleus in dorsal part & ventral nucleus in ventromedial part – ventral nucleus is subdivided into anterior, intermediate & posterior groups. -posterior group is further divided into posterolateral & posteromedial groups. -intralaminar nuclei including centromedian nucleus(located in the internal medullary lamina) midline nuclei(periventricular grey on the medial surface) & reticular nuclei on lateral surface.

Connections and Functions of Thalamus: *afferent impulse from a large numbers of subcortical centers converge on thalamus. -exteroceptive & propioceptive impulses ascend to it through medial lemniscus, spinothalamic tract. -visual & auditory impulses reach medial & lateral geniculate bodies -sensation of taste are conveyed through solitarothalamic tract. Although thalamus does not receive direct olfactory impulse they probably reach it through amygdaloid complex.

-visceral information is conveyed from hypothalamus and probably through reticular formation -thalamus also recieves profuse connection from all parts of cerebral cortex, cerebellum and copus striatum -that is why thalamus is regarded as a great intergrating center where information from all these source is brought together. -this information is projected to almost the whole of cerebral cortex through profuse thalamocortical projecion. *efferent projection also reach corpus striatum, hypothalmus & reticular formation

Clinical Anatomy -Lesion of the thalamus cause impairment of all types of sensibilities: joint sense ( posture & passive movements) being the most affected. -Thalamic syndrome is characterized by disturbances of sensations, hemiplegia or hemiparesis together with hyperaesthsia & severe spontaneous pain. Pleasant as well as unpleasant sensations or feelings are exaggerated.

Metathalamus:

-consists of medial & lateral geniculate bodies on each side of midbrain, below thalamus. Medial Geniculate Body: -an oval elevation situated just below the pulvinar of thalamus & lateral to superior colliculus.inferior brachium conncets to inferior colliculus -conncetions of MGB are as follows: Afferents: 1) lateral leminiscus & 2) fibers from both inf. Colliculi Efferents: gives rise to acoustic radiation going to auditory area of cortex(temporal lobe) through sublentiform part of internal capsule. -Fucntions: MGB is last relay station on pathway of auditory impulse to cerebral cortex

Lateral Geniculate Body -small oval elevation situated anterolateral to MGB, below thalamus- overlapped by medial part of temporal lobe and is conneted to superior colliculus by superior brachium. -structures: six layers 1,4 & 6 receive contralateral optic fibers & 1,2 & 3 layers ipsilateral optic fibers.

-Connections: Afferent: optic tract(lateral root) Efferents: gives rise to optic radiation going to visual pathway of cortex through retrolentiform part of internal capsule -Function: LGB is last station of visual pathway to occpital cortex.

Epithalamus: -occupies caudal part of roof of diencephalon & consists of 1) the right & left habenular nuclei each situated beneath the floor of corresponding habenular trigone 2) pineal body or epiphysis cerebri 3) habenular commissure 4) posterior commissure

Habenular Nucleus: -lies beneath floor of habenular trigone- trigone is a small depressed triangular area situated above the superior colliculus & medial to pulvinar of thalamus.-medially it is bounded by stria medullaris thalami & stalk of pineal body. – it forms the part of limbic system.

Pineal Body: -small conical organ projecting backwards & downwards between two superior colliculi.- placed below splenium of corpus callosum.is separated from it by telachoroidea of 3 rd ventricle & contained cerebral veins. -consists of conical body about 8 mm long & stalk or peduncle which divides anteriorly into two laminae separated by pineal recess third ventricle. Superior lamina of stalk contains habenular commissure & inferior lamina contains posterior commissure.

Morphological Significance: -in reptiles pineal body is represented by double structure: anterior part (parapineal organ) develops into pineal or parietal eye. Posterior part is glandular in nature. In human pineal body represents posterior glandular part only. Structure: -composed of two types of cells: pinealocytes & neuroglial cells, with rich network of blood vessels & sympathetic fibers

Functions: -has long been regarded as a vestigial organ of no importance. -recent investigations has shown that it is an endocrine gland of great importance. -produced hormones that may have regulatory influence on many other endocrine organs like adenohypophysis, neurohypophysis, parathyroid, the adrenal cortex & medulla & gonads) -best known hormone is melatonin which causes changes in skin colour.

Hypothalamus: -part of diencephalon- lies in floor & lateral wall of 3rd ventricle -designated as head ganglion of ANS because it takes part in control of many visceral & metabolic activities of body -anatomically, it includes floor of 3rd ventricle or structure in interpeduncular fossa & lateral wall of 3rd ventricle below hypothalmic sulcus.

Boundaries: -bounded anteriorly by posterior perforated substance & on each side by optic tract & crus cerebri -in saggital section, is bounded anteriorly by lamina terminalis. -inferiorly by floor of 3rd ventricle (from optic chiasma to posterior perforated substance) & posterosuperiorly by hypothalamic sulcus.

Parts of Hypothalamus: -is subdivided into optic part, tuberal & mamillary parts. Nuclei of each part are as follows: Optic part: * supraoptic nuc., above optic chiasma * paraventricular nuc. Just above supraoptic nuc. Tuberal part * ventromedial nuc. * dorsomedial nuc. * tuberal nuc., lateral to ventromedial nuc.

Mamillary part *posterior nuc.,caudal to ventromedial and dorsomedial nuc. *lateral nuc., lateral to posterior nuc.

Important Connections: -Afferent: receives visceral sensations through spinal cord & brain stem(reticular formation)- also conncected to several centers connected with olfactory pathways including piriform cortex: with cerebellum: & with retina -Efferents: *Surpaopticohypophyseal tract from optic nuclei to pars posterior, pars tuberalis & pars intermedia of hypophysis cerebri. *mamillothalamic tact. *mamillotegmental tract (periventricular system of fibers)

Functions of Hypothalamus: -complex neuroglandular mechanism concerned with regulation of visceral and vasomotor activities of body. Functions are as follows: *Endocrine control: by forming releasing hormones & releasing inhibiting hormones regulates TSH, ACTH, STH, LH,FSH & MSH by pars anterior of pituitary gland. -Neurosecretion: oxytocin & vasopressin are secreted by hypothalamus & transported to infundibulum & post. lobe of pituitary gland

- General Autonomic Effect: controls cardiovascular, respiratory & alimentary fx. -Temperature Regulation: balance between heat production & heat loss of body -Regulation of Food & Water Intake: hunger or feeding center is placed laterally, satiety center medially, thirst or drinking center is situated in lateral part of hypothalamus -Sexual Behaviour & Reproduction: through its control of anterior pituitary it controls gametogenesis,various reproductive cycles & maturation & maintenane of secondary sexual characteristics.

-Biological Clocks: Sleep & wakefullness is an example of circadian rhythm. – wakefullness maintained by reticular formation & sleep is produced by hypnogenic zones of thalamus & hypthalamus. Leseion in anterior hypothalamus cause disturbance in this cycle. -Emotion, Fear, Rage, Aversion, Pleasure & Reward:- these facilities are controlled by hypothalamus, limbic system & prefrontal cortex.

Clinical Anatomy Lesion in the hypothalamus give rise to one of the fear following syndrome. 1) Obesity. Frolich’s syndrome or Laurence Moon-Bield syndrome. 2) Diabetes insipidus 3) Diencephalic autonomic epilepsy: characterised by flushing, sweating, lacrimation, tachycardia, retardation of respiratory rate, unconsciousness. 4) Sexual disturbance. Either precocity or impotence 5) Disturbance of sleep. 6) Hyperglycemia & Glycosurea 7) Acute ulceration in the upper part of GI tract.

Subthalamus: -lies between midbrain & thalamus, medial to internal capsule & globus pallidus. Consists of followings: Grey matter: cranial ends of red nucleus & substantia nigra extend into it., subthalamic nucleus & zona incerta. White matter: cranial ends of lemnisci, lateral to red nucleus. Dentothalamic tract along with rubrothalamic fibers, ansa lenticularis, fasciculus lenticularis & subthalamic fasciculus.

-subthalamic nucleus is biconvex in coronal section & situated to dorsolateral to red nucleus & ventral to zona incerta. -zona incerta is a thin lamina of gray matter situated between thalamus & subthalamic nucleus. It is continuous with reticular nucleus of thalamus. Functional singificance is unknown. -lesion of subthalamic nucleus can result in hemiballism characterised by involuntary movements on opposite side of body.